The present invention relates to a room temperature-activating polymer electrolyte fuel cell used in portable power sources, power sources for electric vehicles, and domestic co-generation systems.
Polymer electrolyte fuel cells simultaneously generate electric power and heat through electrochemical reactions of a hydrogen-containing fuel gas and an oxygen-containing oxidant gas, such as the air.
To manufacture this fuel cell, a catalyst layer mainly composed of carbon powder with a platinum metal catalyst carried thereon is formed in first on both faces of a polymer electrolyte membrane that selectively transports hydrogen ions. Subsequently, a gas diffusion layer having both gas permeability and electron conductivity relative to either the fuel gas or the oxidant gas is formed on outside each catalyst layer, giving an electrode composed of the catalyst layer and the gas diffusion layer. The joint body of the electrodes and the electrolyte membrane is referred to as an MEA.
In order to prevent leakage of the supplied gas and mixing of the fuel gas and the oxidant gas, gaskets are disposed across the polymer electrolyte membrane to surround the electrodes. In some cases, the gaskets, the electrodes, and the polymer electrolyte membrane are integrated with one another as the MEA.
Each MEA is interposed between a pair of conductive separator plates, which mechanically fix the MEA and electrically connect the adjoining MEAs with each other in series. A gas flow path is formed in a portion of the separator plate that is in contact with the MEA to feed a supply of the reaction gas to the electrode plane and flow out the produced gas and the remaining excess gas. The gas flow path may be separately attached to the separator plate, but generally a groove is formed on the surface of the separator plate to function as a gas flow path.
The conductive separator plate is required to have high electron conductivity, gas tightness, and high corrosion resistance and the prior art technique thus forms a groove in a dense carbon plate by cutting or another adequate working process to complete the conductive separator plate.
The gas flow path formed in the prior art conductive separator plate generally has multiple linear gas flow paths (straight flow paths) in parallel running from the gas inlet to the gas outlet. In the polymer electrolyte fuel cell, water is produced on the cathode during the operation and, therefore, there is a problem that efficient removal of the product water is required for exertion of the sufficient cell performance. The length of each gas flow path is accordingly extended by decreasing the sectional area of the gas flow path formed in the conductive separator plate and forming a meandering gas flow path (serpentine flow path). This practically raises the gas flow rate and forcibly removes the product water, thereby improving the cell performance.
A plurality of the above single cells are laid one upon another to form the structure of cell stack when fuel cell is used. Heat as well as electric power is generated during the operation of the fuel cell, and a cooling plate is accordingly provided for every one or two single cells in the cell stack, in order to keep the cell temperature at a substantially fixed level and allow utilization of the simultaneously generated thermal energy.
A thin metal plate having the structure that allows circulation of a thermal medium inside thereof, such as cooling water, is generally used for the cooling plate. A flow path for cooling water is formed on the rear face of the conductive separator plate included in the single cell, that is, on a specific face of the conductive separator plate where a cooling water may be flowed, and this conductive separator plate may be used as the cooling plate. At that time, an O-ring or a gasket is required for sealing the thermal medium like cooling water. In this case, the O-ring should be pressed completely to ensure the sufficient electrical conductivity across the cooling plate.
The cell stack requires an aperture called manifold for supplying and discharging the fuel gas and the oxidant gas to the respective single cells. The inner manifold structure, in which an aperture for supplying and discharging the cooling water is formed inside the cell stack, is typically adopted.
In the either of the inner manifold structure and the outer manifold structure, it is necessary that a plurality of single cells including the cooling sections are stackd in one direction to form a stack (cell stack) and a pair of end plates are arranged on both ends of the stack and are fixed with a clamping rod.
In case of clamping by using the clamping rod, it is desirable to evenly clamp the single cells in the plane direction. From the viewpoint of mechanical strength, a metal material such as stainless steel is typically used for the end plates and the clamping rod. These end plates and the clamping rod are electrically insulated from the stack via an insulator, so that the electric current does not leak outside via the end plates. The clamping rod may be inserted in a through hole formed in the separator plates. It is also proposed that the whole stack may be clamped via the end plates with a metal belt.
In the polymer electrolyte fuel cell thus obtained, the electrolyte membrane functions as the electrolyte in the wet state, and the supplies of the fuel gas and the oxidant gas should thus be humidified. In the temperature range up to at least 100xc2x0 C., the higher water content of the polymer electrolyte membrane increases the ion conductivity and lowers the inner resistance of the cell, thus ensuring the high performance.
Supply of a high humid gas at temperatures higher than the cell driving temperature, however, causes sweating inside the cell and the water drops undesirably interfere with the smooth gas supply. Water is produced by power generation on the cathode, to which the supply of the oxidant gas is fed, and thus there is a problem that the efficiency of removal of the product water and the cell performance are lowered. The supply of gas is thus generally humidified to have the dew point a little lower than the cell driving temperature.
As typical method of humidifying the gas supply, there are a bubbler humidification process that bubbles the supply of gas in deionized water kept at a predetermined temperature for humidification and a membrane humidification process that makes a flow of deionized water kept at a predetermined temperature in one face of a membrane that allows easy transfer of the water content, such as an electrolyte membrane, while making a flow of the gas supply in the other face for humidification. When a gas obtained by steam reforming a fossil fuel like methanol or methane is used for the fuel gas, steam is included in the reformed gas and humidification is thus not required in such cases.
The humidified fuel gas and oxidant gas are supplied to the polymer electrolyte fuel cell for power generation. There is a current density distribution in a single plane of an arbitrary single cell in the cell stack. The fuel gas is humidified and then fed into the fuel cell via a gas inlet and power generation consumes hydrogen included the fuel gas. This leads to a phenomenon of a higher hydrogen partial pressure and a lower steam partial pressure in the upper stream of the gas flow path and a lower hydrogen partial pressure and a higher steam partial pressure in the lower stream of the gas flow path.
The oxidant gas is humidified and then fed into the fuel cell via a gas inlet and power generation consumes oxygen included in the oxidant gas, while producing water. This leads to a phenomenon of a higher oxygen partial pressure and a lower steam partial pressure in the upper stream of the gas flow path and a lower oxygen partial pressure and a higher steam partial pressure in the lower stream of the gas flow path.
The temperature of cooling water used for cooling down the cell is lower in the vicinity of the inlet and the higher in the vicinity of the outlet and there is accordingly a temperature distribution in a single cell plane. Because of the above reasons, there is a current density distribution (performance distribution) in a single cell plane.
When the uniform hydrogen partial pressure and steam partial pressure in the fuel gas, the uniform oxygen partial pressure and steam partial pressure in the oxidant gas, and the temperature distribution in a single cell plane are extremely enhanced and deviated from the optimum conditions, there is an extremely dried state (overdried state) or extremely wet state (overflooding state). Both the overdried state and the overflooding state may be observed in a single cell plane. This causes not only the current density distribution but malfunction of the cell.
In the cell stack including a large number of single cells, when the above problem arises in part of the many single cells in the cell stack, the deteriorating single cells interfere with the smooth operation of the whole cell stack. When a the single cell in the cell stack falls into the overflooding state, the pressure loss for the gas supply increases in the single cell. The gas supply manifold connects with the respective single cells in the cell stack and the overflooding state in one single cell accordingly interferes with a smooth flow of the gas and leads to the overflooding state in other single cells, and this undesirably accelerates the overflooding state.
When a single cell in the cell stack falls into the overdried state, on the other hand, the pressure loss for the gas supply decreases in the single cell. The overdried state in the single cells accordingly facilitates the gas flow and this undesirably accelerates the overdried state.
Such problems may be ascribed to the higher steam partial pressure in the gas in the vicinity of the gas outlet than that in the vicinity of the gas inlet both on the anode that receives a supply of the fuel gas and on the cathode that receives a supply of the oxidant gas.
When the polymer electrolyte fuel cell is used as the power supply for an electric vehicle, the requirements are the sufficient size reduction, weight reduction, and cost reduction as well as the high response under the high output conditions, that is, the high current-voltage characteristic in the high current density range. It is accordingly desirable to avoid the overflooding state or the overdried state discussed above.
The electrodes (the anode and the cathode) in the polymer electrolyte fuel cell generally have catalyst layers formed on both faces of the polymer electrolyte membrane and gas diffusion layers formed on the outside surfaces of the catalyst layers. The gas diffusion layers mainly have the three functions discussed below.
The first function is diffusion of the gas to allow a uniform supply of the fuel gas or the oxidant gas from the flow path located on the outside surface of the gas diffusion layer to the catalyst in the catalyst layer. The second function is quick discharge of water produced in the catalyst layer to the gas flow path. The third function is transmission of electrons required for the reaction or produced through the reaction.
The gas diffusion layer accordingly requires the high reaction gas permeability, steam permeability, and electron conductivity. The prior art technique adopts the gas diffusion layer of the porous structure for the enhanced gas permeability, dispersion of a water-repellent polymer such as a fluorocarbon resin in the gas diffusion layer for the enhanced steam permeability, and the gas diffusion layer composed of an electron conductive material such as carbon fibers, metal fibers, or carbon powder for the enhanced electron conductivity.
The above techniques for the enhanced gas permeability, steam permeability, and electron conductivity, however, exert the conflicting effects. For example, the raised porosity of the gas diffusion layer by reducing the diameter of carbon fibers or decreasing the quantity of carbon fibers for the enhanced gas permeability results in lowering the electron conductivity. Addition of the water-repellent polymer for the enhanced steam permeability results in lowering the gas permeability and the electron conductivity. Some proposed method does not use the gas diffusion layer composed of a single material but, for example, combines a layer of carbon fibers with another layer of carbon powder and a water-repellent polymer to make the conflicting functions compatible with one another. There has been, however, no proposed technique that specifies and proves the diverse characteristics of the carbon powder required for the gas diffusion layer.
The electrode including carbon powder with a noble metal catalyst carried thereon and a conductive porous base material of the gas diffusion layer is typically obtained by mixing carbon powder with a noble metal carried thereon and an organic solvent, such as isopropyl alcohol to obtain an ink and applying this ink onto the base material by screen printing or transfer method. Another process prepares a slurry including catalyst powder and applies the slurry on a resin sheet by the doctor blade method.
It is sometimes proposed to admix carbon powder with polytetrafluoroethylene (PTFE) carried thereon into the ink to enhance the water repellency of the electrode.
In order to prevent deterioration of the characteristic due to the current distribution in the electrode plane, the effective measure is to vary the water repellency of the electrode either in the plane direction of the electrode or in the direction of gas diffusion (thickness). Especially in the vicinity of the outlet of the gas flow path in the electrode, the pressure of the supplied gas tends to be lower than the pressure in the vicinity of the inlet to decrease a relative humidity and cause the dry state, and it is accordingly desirable to heighten the water holding property of the MEA from the inlet to the outlet.
The prior art screen printing or doctor blade method, however, does not allow a variation in composition of the electrode in the plane direction and requires multiple coating even in the direction of thickness, which undesirably complicates the manufacturing process.
In order to solve the problems of the prior art discussed above, an object of the present invention is, in first, to provide an improved polymer electrolyte fuel cell having a high discharge characteristic or more specifically a high current-voltage characteristic in a high current density range by varying the degree of water repellency of an anode and a cathode according to the position. Another object of the present invention is, in second, to provide an improved polymer electrolyte fuel cell having a high discharge characteristic by optimizing water repellency, a specific surface area, a primary particle diameter, and a DBP absorption of carbon powder in an anode and a cathode.
The present invention is directed to a polymer electrolyte fuel cell comprising a single cell having a cathode and an anode that are arranged across a hydrogen ion-conductive polymer electrolyte membrane, and a pair of conductive separator plates that are disposed outside the anode and the cathode and have gas flow paths for supplying and discharging a fuel gas and an oxidant gas to and from the anode and the cathode respectively, wherein each of the cathode and the anode comprises catalyst particles, a hydrogen ion-conductive polymer electrolyte, a conductive porous base material, and a water repellent agent, and a water repellency of at least one of the cathode and the anode varies in a direction of thickness or in a plane direction.
It is preferable that the water repellency varies continuously, i.e., the degree of the water repellency is gradient.
In this application, it is effective that the water repellency of the conductive porous base material varies in the direction of thickness and is higher on a side of the conductive separator plate than on a side of the hydrogen ion-conductive polymer electrolyte membrane.
It is also effective that the water repellency of the conductive porous base material varies in the plane direction and is higher on a gas outlet side of the separator than on a gas inlet side of the separator.
The present invention is also directed to another polymer electrolyte fuel cell comprising a single cell having a cathode and an anode that are arranged across a hydrogen ion-conductive polymer electrolyte membrane, and a pair of conductive separator plates that are disposed outside the anode and the cathode and have gas flow paths for supplying a fuel gas and an oxidant gas to the anode and the cathode, respectively, wherein each of the cathode and the anode comprises catalyst particles, a hydrogen ion-conductive polymer electrolyte, a conductive porous base material, and a water repellent agent, and a water repellency of the cathode is higher than a water repellency of the anode.
In this application, it is effective that a gas permeability of the conductive porous base material is 1 to 60 second/100 ml as a Gurley constant.
It is also effective that a gas permeability of the conductive porous base material in the cathode is 1.2 to 2.0 times a gas permeability of the conductive porous base material in the anode.
It is effective that a porosity of the conductive porous base material in the cathode is 1.2 to 2.0 times a porosity of the conductive porous base material in the anode.
It is further effective that a thickness of the conductive porous base material in the cathode is 1.2 to 3.0 times a thickness of the conductive porous base material in the anode.
In the polymer electrolyte fuel cell discussed above, it is effective that each of the cathode and the anode comprises a catalyst layer that is joined with the hydrogen ion-conductive polymer electrolyte membrane, and a gas diffusion layer that is in contact with the conductive separator plate, the catalyst layer is mainly composed of catalyst particles carried on carbon particles and a hydrogen ion-conductive polymer electrolyte, the gas diffusion layer is mainly composed of a conductive porous base material containing carbon particles, and a water repellency of the carbon particles included in the gas diffusion layer is higher than water repellency of the carbon particles included in the catalyst layer, and the water repellency of at least one of the cathode and the anode varies in the direction of thickness.
In this embodiment, it is effective that the carbon particles included in the gas diffusion layer is disposed at a joint between the catalyst layer and the conductive porous base material.
It is also effective that a specific surface area of the carbon particles included in the gas diffusion layer is not greater than xc2xd a specific surface area of the carbon particles included in the catalyst layer.
It is effective that a primary particle diameter of the carbon particles included in the gas diffusion layer is not less than 1.3 times a primary particle diameter of the carbon particles included in the catalyst layer.
It is further effective that a DBP absorption of the carbon particles included in the gas diffusion layer is not greater than ⅔ a DBP absorption of the carbon particles included in the catalyst layer.